Method for fabricating lanthanum boride cathodes

ABSTRACT

A method for fabricating cathodes with a lanthanum boride coating is disclosed in which the coating is applied to any base metal by spraying with a plasma flame spray. One plasma source is a spray gun utilizing an electric arc discharge through which a plasma gas is passed so that a plasma of ionized gas issues from the spray gun into which is injected a carrier gas having lanthanum boride powder suspended therein. The high-temperature plasma flame appearing at the nozzle of the spray gun melts the lanthanum boride (LaB6) almost instantly and the droplets formed thereby are carried to the surface of a prepared base material.

2,659,685 11/1953 Lafferty States atent Louis J. Favreau Elnora, N.Y.

Apr. 30, 1969 Dec. 28, 1971 General Electric Company Inventor Appl. No.Filed Patented Assignee METHOD FOR FABRICATING LANTHANUM BORIDE CATHODESn aw/m OTHER REFERENCES Encyclopedia of Science & Technology Vol. 8,1960 Mc- Graw-l-lill Primary Examiner-William L. Jarvis Attorneys-JohnF. Ahem, Paul A. Frank, Jerome C.

Squillaro, Frank L. Neuhauser, Oscar B. Waddell and Forman Joseph B.

ABSTRACT: A method for fabricating cathodes with a lanthanum boridecoating is disclosed in which the coating is applied to any base metalby spraying with a plasma flame spray. One plasma source is a spray gunutilizing an electric arc discharge through which a plasma gas is passedso that a plasma of ionized gas issues from the spray gun into which isinjected a carrier gas having lanthanum boride powder suspended therein.The high-temperature plasma flame appearing at the nozzle of the spraygun melts the lanthanum boride (LaB almost instantly and the dropletsformed thereby are carried to the surface of a prepared base material.

METHOD FOR FABRICATING LANTI'IANIJM BORIDE CATI'IODES BACKGROUND OF THEINVENTION This invention relates generally to improved cathodes forelectron discharge devices and more particularly pertains to a methodfor making an improved cathode by the application of lanthanum boride.

The borides of the alkaline earth and rare earth metals and thorium havebeen found to improve the thermionic emission characteristics of cathodematerials such as nickel, tungsten, tantalum, molybdenum, and rheniumwhen such metals are coated with the aforementioned borides. Thedesirable properties of the rare earth metals are well known in the artand are fully discussed, for example, in US. Pat. No. 2,639,399 to J. M.Lafferty. As disclosed therein, lanthanum boride, LaB, is particularlydesirable for electron emitters. One of the difiiculties previouslyencountered in utilizing lanthanum boride however is that it did notadhere readily to cathode base metals. Accordingly, there was a seriousneed for the development of a process for applying lanthanum boride (LaBto cathode base metals in such a manner that the LaB would adhere to thebase metal. The present invention describes such a process.

SUMMARY OF THE INVENTION Briefly, the present invention pertains to amethod for fabricating lanthanum boride cathodes including the steps ofcleaning the base cathode metal by sandblasting techniques and sprayingthe base metal with molten lanthanum boride which may conveniently beobtained from a plasma flame spray gun operating with a SOO-ampere-arccurrent and with plasma gas passed through the arc to create a plasmaflame into which is injected lanthanum boride powder suspended in agaseous carrier. The resulting spray issuing from the nozzle of thespray gun is pennitted to strike the base metal to form a coatingthereon. This coating is not only harder and more tenacious than anyprior art coatings, but also produces a cathode with unusually higherelectron emission characteristics than lanthanum boride cathodes made byconventional methods such as painting, vacuum sintering and cataphoreticdeposition.

It is therefore an object of this invention to provide a method forapplying lanthanum boride to cathode base metals so that a harder andmore tenacious bond exists therebetween.

It is another object of this invention to provide a lanthanum boridecoating to base metals which does not mar or chip when scraped with asharp instrument and is not damaged when the base metal with thelanthanum boride coating is bent, twisted, or cut into strips.

It is a further object of this invention to provide a lanthanum boridecathode which exhibits higher thermionic emission characteristics thanthose made by conventional methods.

Other attendant advantages and features of this invention will be betterunderstood from the following description taken in connection with theaccompanying drawing. The scope of the invention will be moreparticularly pointed out in connec tion with the appended claims.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTReferring now to FIG. l of the drawing, there is illustrated a plasmaflame spray gun 1] for producing lanthanum boride coatings in accordancewith the invention. Spray guns found suitable for this purpose aremarketed by Metco of Westbury, Long Island, NY. and are designated bytype Nos. 2M and 3MB.

The plasma flame spray gun It provides a plasma flame 12 which can becontrolled over a wide temperature range so as to ensure the proper melttemperature for the lanthanum boride. The heat for spraying is generatedby an electric arc of extremely high intensity and is transferred to thematerial to be sprayed by a plasma gas which is brought to the spray gun11 through a gas line 13 which terminates at a point behind the electricarc. The plasma gas, being applied under pressure, drives the electricare through the nozzle of the spray gun Ill and directs the flame 12toward the material to be sprayed.

The plasma gas consists essentially of a primary gas selected from thegroup consisting of nitrogen, argon, and helium and a secondary gas ofhydrogen. The primary gas is mixed with from S to 15 percent by volumeof the secondary gas to form the desired plasma gas.

The electric are utilized in the spray gun I 1 is provided by a directcurrent which is supplied from a rectifier type power supply unitthrough electrical conductors l4 and 15. As a result of the hightemperatures created by the plasma flame, it is necessary to provide acirculating coolant through the nozzle of the spray gun. This coolant,which may be water, for example, is conducted to and away from thenozzle by a coolant line to appropriately connected to a heat exchanger.

The lanthanum boride is brought to the spray gun 11 in the form of apowder suspended in a carrier gas stream which is taken from the primarygas nitrogen, argon, or helium. The lanthanum boride suspended in thecarrier gas stream is brought to the nozzle of the spray gun I] by aline 17. The lanthanum boride thus introduced into the nozzle of thespray gun 11 is melted by the flame 12 and the droplets formed therebyare propelled to the surface being coated by the blast of plasma gas asit rushes through the nozzle orifice.

As illustrated in FIG. I, the plasma flame 12 is directed toward stripsof cathode base material such as, for example, tantalum, tungsten,nickel, rhenium, or molybdenum. The strips 18 are supported by a supportmember 19 which is held in a fixed position by a vise 20. The strips 18,the support member 19, and vise are all enclosed in a hood 21 whichcollects the hot gases and rents them to the outside air.

Having thus described apparatus useful in practicing the presentinvention, the method for depositing lanthanum boride on base metalcathodes will now be described. Assuming that it is desired to make acathode of lanthanum boride coated upon a rhenium base having aconfiguration such as illustrated in FIG. 2, a piece of rhenium metal isselected so that cathode base strips can be subsequently cut therefrom.Before cutting and spraying however, the surface of the rhenium must beprepared properly. This is accomplished by washing the metal thoroughlyin a solvent and then blasting the metal with an abrasive such asalumina with a particle size of 325-mesh at an air pressure of 7080 psi.The degree of roughening produced by the abrasive blasting naturallydepends upon the type and mesh size of the abrasive, the air pressure,and the hardness of the surface. The foregoing air pressure and theparticle size were found acceptable for the making of lanthanum boridecoated cathodes, but are indicated solely for the purpose ofillustration and are not meant by way of limitation.

After preparing the surface of the rhenium, the rhenium metal is thencut into strips of the size desired for the finished cathode.Considerable care must be exercised so as not to contaminate the surfaceof the rhenium after it is cleaned. The strips 18 thus out are placed orformed on the support member R9 in preparation for the plasma spraying.

The best lanthanum boride coatings have been achieved when the plasmagas has been formulated in approximate proportions of percent to 15percent by volume of argon gas to hydrogen gas, respectively. Argon hasbeen found to be preferable to nitrogen since nitrogen reacts with thelanthanum to cause nitriding which adversely affects the emissioncharacteristics of the resultant cathode if exposed to moisture in theair. Helium, however, produces no such affects and could be usedinterchangeably with argon. The means for achieving this proportion orratio of gases can readily be achieved by measuring the individual flowrates of the two gases into a unitary gas line. Such apparatus is a partof the prior art since it is found in the Metco type 2M and 3Mb plasmaspray guns with which the present invention may be practiced, andaccordingly need not be discussed further.

The purpose of the hydrogen in the plasma gas is to cause burning withthe atmospheric oxygen in the area surrounding the plasma spray so as toprovide more protection for the lanthanum boride droplets fromundesirable chemical action than that afforded by the presence of theinert gas alone.

As described previously, the primary gas is also utilized as a carriergas for the lanthanum boride powder. This is accomplished by introducinga portion of the primary gas into a powder feed unit (not shown) inwhich lanthanum boride powder is contained. The size of the lanthanumboride powder may range between 5 to 150 microns with very good resultsin the final coating; however, it is preferable to use lanthanum boridepowder in the 5- to lO-micron size for the coatings described herein.The powder contained in the powder feed unit is agitated so that as theprimary gas passes therethrough, the lanthanum boride powder is pickedup and conveyed to the spray gun 11 through the gas line 17.

While various arc currents have been employed in the coating process,the most desirable results have been achieved when the are current forthe spray gun is approximately 500 amperes. With this are current andthe plasma gas supplied to the spray gun in the aforementionedproportions, the lanthanum boride powder introduced into the gas streamis melted by the plasma flame 12 and the droplets formed thereby arepropelled to the surface of the rhenium strips. As illustrated in FlG.l, the plasma flame spray gun 22 is positioned or spaced from therhenium strips by a distance such that the tip or point of the plasmaflame 12 just strikes the surface of the rhenium strips. in this way,minimum heating of the base metal occurs and the best coating resultsare obtained.

FIG. 3 illustrates a cross-sectional view of a cathode strip 18 with acoating 22 of lanthanum boride applied in accordance with the instantinvention. As described previously, the lanthanum boride coating appliedby the foregoing process is much harder and forms a more tenacious bondwith the base material than by any of the prior art processes. Anadditional advantage of the lanthanum boride coating applied inaccordance with the instant invention resides in the higher thermionicemission characteristics of the resultant cathode which exceed those oflanthanum boride cathodes produced by conventional techniques. The exactreason for this improvement is not clearly understood; however, it maybe a result of the existence of free lanthanum or possibly a result ofan increase in surface area of emission.

Additionally, it has been found that cathodes made in accordance withthe present invention are almost completely activated, i.e., capable ofemission, apparently because some slight decomposition of the lanthanumboride takes place which releases free lanthanum metal. This has theadditional advantage of eliminating the requirement of high-temperatureactivation and therefore may permit the use of tantalum and other metalsas base metals for cathodes where lower emission characteristics can betolerated.

While the invention has been described in connection with a specifictype of plasma device, namely, a plasma flame spray gun, obviously otherplasma spray means may likewise be utilized. Similarly, whereas theinvention has been described with regard to cathode strips, obviouslyother configurations such as wires, cylinders, rods, discs, or any othershaped cathode may be used. Therefore, it is intended that the followingclaims include all such variations as fall within the true spirit andscope of this invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A method for making a lanthanum-bonde-coated cathode having improvedthermionic emission characteristics and a tenacious bond between thelanthanum boride and a cathode base metal, said method comprisingspraying molten lanthanum boride on the surface of said cathode basemetal.

2. The method for making a lanthanum-boride-coated cathode as recited inclaim 1 wherein the step of spraying comprises:

formulating a plasma gas with hydrogen and a gas selected from the groupconsisting of argon, nitrogen, and helium,

introducing the plasma gas into an electric arc to create a plasma ofionized gas, and

injecting lanthanum boride powder into the plasma of ionized gas wherebythe heat created by the electric arc melts the lanthanum boride.

3. The method for making a lanthanum-boride-coated cathode as recited inclaim 2 further comprising the step of:

cleaning the surface of the metal to be sprayed by washing with asolvent and blasting with an abrasive.

4. The method for making a lanthanum-boride-coated cathode as recited inclaim 3 wherein the step of injecting lanthanum boride powder into theplasma of ionized gas further comprises suspending the lanthanum boridepowder in a carrier gas selected from the group consisting of argon,nitrogen, and helium.

5. The method for making a lanthanum-boride-coated cathode as recited inclaim 2 wherein the step of formulating a plasma gas comprises:

mixing 5 to 15 percent of hydrogen with 95 to percent by volumerespectively, of a gas selected from the group consisting of argon,nitrogen, and helium.

6. The method of claim 1 wherein the cathode base metal is selected fromthe group consisting of tungsten, rhenium, tantalum, nickel, andmolybdenum.

k i t

1. A method for making a lanthanum-boride-coated cathode having improvedthermionic emission characteristics and a tenacious bond between thelanthanum boride and a cathode base metal, said method comprisingspraying molten lanthanum boride on the surface of said cathode basemetal.
 2. The method for making a lanthanum-boride-coated cathode asrecited in claim 1 wherein the step of spraying comprises: formulating aplasma gas with hydrogen and a gas selected from the group consisting ofargon, nitrogen, and helium, introducing the plasma gas into an electricarc to create a plasma of ionized gas, and injecting lanthanum boridepowder into the plasma of ionized gas whereby the heat created by theelectric arc melts the lanthanum boride.
 3. The method for making alanthanum-boride-coated cathode as recited in claim 2 further comprisingthe step of: cleaning the surface of the metal to be sprayed by washingwith a solvent and blasting with an abrasive.
 4. The method for making alanthanum-boride-coated cathode as recited in claim 3 wherein the stepof injecting lanthanum boride powder into the plasma of ionized gasfurther comprises suspending the lanthanum boride powder in a carriergas selected from the group consisting of argon, nitrogen, and helium.5. The method for making a lanthanum-boride-coated cathode as recited inclaim 2 wherein the step of formulating a plasma gas comprises: mixing 5to 15 percent of hydrogen with 95 to 85 percent by volume respectively,of a gas selected from the group consisting of argon, nitrogen, andhelium.
 6. The method of claim 1 wherein the cathode base metal isselected from the group consisting of tungsten, rhenium, tantalum,nickel, and molybdenum.